Heat-developable image-recording materials that can produce images by using the heat development process have been known, and disclosed in, for example, U.S. Pat. Nos. 3,152,904, 3,457,075, and "Imaging Processes and Materials", Neblette's 8th edition, pp.279-291 (1969). The image-recording materials disclosed in the aforementioned literature comprise a silver source which can be reduced (for example, organic silver salt), a catalytically active amount of photocatalyst (for example, silver halide), a color-tone adjustor for controlling tonality of silver, and a reducing agent, which are dispersed in a binder. Such heat-developable image-recording materials are stable at an ambient temperature, but they form blackened silver through oxidation-reduction reaction of the reducible silver salt and the reducing agent when they are heated to an elevated temperature (e.g., 120.degree. C.) after light exposure. This reaction is promoted by catalytic action of latent image generated by the light exposure.
As another image-forming scheme, when the materials do not contain the catalytic amount of silver halide, blackened images can also be obtained by imagewise tracing at an elevated temperature on the materials with a thermal head.
Such image-formation methods require no processing solution such as a developer, and provide images only by heating. The methods do not generate sulfite gas, ammonia gas and the like, and therefore, the materials have been focused as recording materials used in image-forming apparatuses utilizing laser rays. Laser image-forming apparatuses have been used in various fields such as image-forming apparatuses for medical use, photomechanical reproduction, and other industrial use.
These heat-developable recording materials generally require heating at a temperature of 110.degree. C. or higher for 10 seconds to 60 seconds. Output speeds have become faster with the progress of laser image-forming apparatuses, and it has been desired to improve sensitivity and developing speed of the recording materials. In recent years, especially as heat-developable recording materials for photomechanical reproduction, materials utilizing infectious development with an ultrahigh contrast agent have been developed. However, since the infectious development requires prolonged heating time, it has been desired to realize more faster developing speed. Since fog is generally enhanced when the development temperature is elevated to obtain a faster developing speed, an increase of the developing temperature is limited. Therefore, there has been desired a heat-developable recording material that can be developed at a high developing speed within a temperature range that does not enhance the fog.
As conventional high contrast agents for producing high contrast images, there have been disclosed, for example, acylhydrazine derivatives (U.S. Pat. Nos. 5,464,738, 5,512,411, 5,496,695, and 5,536,622), acrylonitrile derivatives (U.S. Pat. Nos. 5,545,516 and 5,635,339), malondialdehydes (U.S. Pat. No. 5,654,130), isoxazoles (U.S. Pat. No. 5,705,324) and the like. As method for accelerating development processes, uses of amine compounds (U.S. Pat. No. 5,545,505), hydroxamic acids (U.S. Pat. No. 5,545,507), hydrogen donors (U.S. Pat. No. 5,637,449) and the like are disclosed.
However, they are still insufficient to achieve a desired high developing speed, and a further effective means has been desired.